This project proposes a comprehensive approach to understanding the chemistry and biology of aromatic ring-hydroxylating dioxygenases from the soil bacteria, Acinetobacter sp. ADP1. These enzymes catalyze the (cis) dihydroxylation of aromatic rings and, in so doing, incorporate both atoms of O2 into the ring. The proposal addresses fundamental questions for this class of enzymes including catalytic mechanism(s), the roles of the metal centers, and the molecular determinants of substrate preferences, and will investigate a previously uncharacterized aromatic ring-hydroxylating enzyme, anthranilate 1,2-dioxygenase (Ant) comparing catalytic and structural properties to those of the related benzoate 1,2-dioxygenase (Ben) from the same organism. Both of these are 2-component systems consisting of a A3B3 hexameric oxygenase containing a Rieske iron center and a mononuclear ferrous center in the A subunit, and a reductase protein containing flavin and Fe/S centers. The mononuclear center carries out the oxygenase reaction while the Rieske center acts as a redox carrier. In order to address the question of substrate specificity, the principal investigator proposes to use an approach consisting of random mutation and in vivo selection to isolate Ben enzymes that have been modified for the utilization of anthranilate as an efficient substrate. Comparison of amino acid sequences of several mutated dioxygenases resulting from this procedure with those of the wild-type enzymes, in conjunction with structural and spectroscopic data will allow an understanding of the features responsible for substrate specificity. A catalytic mechanism for these enzymes and several tests of the mechanism are also proposed.